1. Field of the Invention
The present invention relates to an electric double layer capacitor and, more particularly, to a large-capacitance electric double layer capacitor using solid-state polarized electrodes as electrodes.
2. Description of the Prior Art
The first prior art of an electric double layer capacitor will be described below with reference to FIGS. 1 to 3.
FIG. 1 is a perspective view showing the structure of the conventional electric double layer capacitor. FIG. 2 is a cross-sectional view of a basic cell of the conventional electric double layer capacitor. FIG. 3 is a sectional view taken along a line III--III in FIG. 1.
Referring to FIG. 2, a pair of polarized electrodes 3 are made from solid-state activated carbon such as activated carbon/polyacene-based material described in Japanese Unexamined Patent Publication No. 4-288361. A pair of collectors 4 on the two sides are made from conductive carbon-containing rubber or plastic and contact-bonded to the polarized electrodes 3. The polarized electrodes 3 oppose each other via a separator 1 made from a porous material. The polarized electrodes 3, a gasket assembly 5 having a frame-like structure in which a vent valve 8 is formed as shown in FIG. 3, and the collectors 4 together form a basic cell 7 (FIG. 1) in which an electrolytic solution 6 is sealed.
Since the breakdown voltage of this electric double layer capacitor is restricted by the electrolytic voltage of the electrolytic solution 6, a plurality of basic cells 7 are connected in accordance with a required breakdown voltage. Additionally, to decrease the connection resistance between adjacent basic cells 7 and between the basic cell 7 and a terminal electrode 13 (FIG. 1), the electric double layer capacitor is pressurized by a predetermined pressure in the stacking direction by a pair of pressurizing plates 12 as shown in FIG. 1.
Recently, new applications of such an electric double layer capacitor have been found and studied because the use of the polarized electrodes 3 has increased the capacitance and decreased the ESR (Equivalent Series Resistance). For example, an electric double layer capacitor is used in combination with a lead-acid battery to form a starter motor driving power supply of an automobile or used in combination with a solar cell to form an auxiliary power supply. In either application, it is highly possible that an electric double layer capacitor is installed in a high-temperature environment. Therefore, it is essential to guarantee the reliability in such an environment.
Unfortunately, if the conventional electric double layer capacitor as shown in FIG. 1 is placed in a high-temperature environment for a long time period, the solvent in the electrolytic solution 6 held inside the capacitor reduces by evaporation. Consequently, the capacitance decreases or the ESR increases, and this lowers the reliability.
As one method of solving this problem, an electric double layer capacitor as shown in FIG. 4 is disclosed in Japanese Unexamined Patent Publication No. 6-20875. In the electric double layer capacitor described in this publication, an air gap formed between a gasket assembly 5 and polarized electrodes 3 is filled with a gel electrolytic solution 14 or an electrolytic solution holding member such as quartz wool or a water-absorbing polymer containing an electrolytic solution equivalent to the electrolytic solution 14. This electrolytic solution holding member compensates for a decrease of an electrolytic solution 6 by evaporation in the electric double layer capacitor. This is the second prior art.
The first problem in the above two prior arts is that the effect of preventing the evaporation of the electrolytic solution 6 is unsatisfactory and makes the reliability difficult to improve.
The reason for this is as follows. The electric double layer capacitor described in Japanese Unexamined Patent Publication No. 4-288361 has a closed structure in which the vent valve 8 is filled and sealed with a resin. To prevent the evaporation of the electrolytic solution 6, it is necessary to improve the sealing properties. In this structure, however, a gas is produced by a redox reaction upon application of a voltage and the internal pressure is raised by a temperature rise. Consequently, a cell stacked body 11 expands and breaks within short time periods.
To solve this problem, it is possible to form a structure in which the vent valve 8 is allowed to communicate with the outside without being filled with an adhesive. If this is the case, however, the following problems are posed by the individual fillers, so the reliability is still difficult to improve.
First, if the filler is the gel electrolytic solution 14 as in the first and second prior arts, after the cell stacked body 11 is assembled, the gel electrolytic solution 14 is injected from the vent valve 8 into the air gap between the polarized electrodes 3 and the gasket assembly 5, thereby forming an electric double layer capacitor. The viscosity of the gel electrolytic solution 14 is initially high and increases with time. Accordingly, it is difficult to uniformly fill the whole small narrow air gap with the gel electrolytic solution 14. Consequently, the electrolytic solution 6 evaporates from a portion left behind as an air gap without being filled, and the reliability lowers.
Second, consider a case in which the filler is quartz wool or a water-absorbing polymer. A formation method in this case is to assemble the cell stacked body 11, fill the air gaps with quartz wool or a water-absorbing polymer, and inject the gel electrolytic solution 14 or the electrolytic solution 6.
It is difficult to inject the gel electrolytic solution 14 for the same reason as in the first case. If the electrolytic solution 6 is, e.g., sulfuric acid, this electrolytic solution 6 can be injected with no problem. However, since this easy injection increases the moisture release, no remarkable effect on dry-up can be obtained. Although the reliability is slightly improved because the electrolytic solution holding member is formed around the electrolytic solution 6, this effect is not so typical.
The second problem is the difficulty of improving the productivity in forming electric double layer capacitors by the prior arts.
The reason for this is as follows. In the electric double layer capacitor described in Japanese Unexamined Patent Publication No. 6-20875, after the capacitor is assembled the entire portion around the polarized electrodes 3 is uniformly filled with the electrolytic solution 6. Accordingly, it is necessary to accurately arrange both of the polarized electrodes 3 in the central portion of the gasket assembly 5. Additionally, as already described above, uniformly filling the air gap with the gel electrolytic solution 14 is difficult and time-consuming. Also, one additional step is required to form the electrolytic solution holding member such as a water-absorbing polymer or quartz wool.
The third problem is that electric double layer capacitors are difficult to miniaturize by the prior arts.
The reason for this is as follows. In the electric double layer capacitor described in Japanese Unexamined Patent Publication No. 6-20875, an air gap with a certain predetermined size or lager must be formed between the polarized electrodes 3 and the gasket assembly 5. This is necessary even if the size of the cell stacked body 11 is decreased, and downsizing is also difficult in the direction of thickness. This makes the electric double layer capacitor difficult to miniaturize.